Improve README

Signed-off-by: Nicolas Iooss <nicolas.iooss_git@polytechnique.org>
Add channels command to the listener
Add tests for the channels command

Signed-off-by: Nicolas Iooss <nicolas.iooss_git@polytechnique.org>
Document "channels" command in README

Signed-off-by: Nicolas Iooss <nicolas.iooss_git@polytechnique.org>